The present invention relates to structures made from multiple layers of sheet material adhered together. The invention relates more particularly to such structures in which the various layers are of dissimilar materials or have different strength properties.
A variety of products are constructed from multiple layers of sheet material joined together by adhesive, referred to herein as laminated structures. Laminated structures can be formed of many different types of rigid or flexible sheet materials. The motivation for making a structure as a laminated structure, as opposed to using a single layer of equivalent thickness, can vary depending on the particular application. For instance, in the case of a laminated wood structure such as plywood, the direction in which the grain runs in the various layers can be alternated between 0xc2x0 and 90xc2x0 directions so that the resulting plywood has similar bending strength in both directions. In contrast, a single wood layer of the same type and thickness would have a significantly lower bending strength in one direction than the other because of the effect of the grain.
In other cases, multiple layers are used because the particular material of which the structure is to be made is available only in sheets whose thickness is substantially smaller than the needed thickness of the structure. For instance, many types of paperboard structures are formed as laminated structures because paperboard generally is not available in thicknesses greater than about one millimeter, whereas the structure to be formed may have to have a thickness of several millimeters or more to meet strength and/or dimensional requirements. As an example, paperboard tubes are manufactured for various uses, including as containers for products, as cores for winding paper, film, and textiles, as concrete forms, as structural members, and others. In many of these types of tubes, dimensional and/or strength requirements dictate that from three to as many as 25 or more layers of paperboard must be used to build the tube. A paperboard tube typically is made by sequentially wrapping a plurality of paperboard plies about a mandrel having the desired shape of the tube. Adhesive is applied to the plies to join them together.
The strength of a paperboard tube depends on a number of factors, and there are several different strength properties one or more of which may be more important than others in a particular application. An important strength property that is assessed and taken into account in the design of paperboard tubes by the assignee of the present application, particularly with respect to paperboard winding cores, is the flat crush strength of the tube. In a flat crush test, the tube is loaded between two flat plates parallel to the axis of the tube. One plate is held stationary and the other plate is moved toward the stationary plate at a defined slow rate, and the load exerted on the tube is continuously monitored. The flat crush strength of the tube is the highest measured load before the tube is crushed.
Another significant strength property of a paperboard tube is the radial crush strength. Radially inward pressure loads can be exerted on tubes in some applications, such as when a web material is wound tightly about a paperboard winding core. Such winding cores must be able to withstand the expected radial crush loads without failing.
The beam bending strength of paperboard tubes can also be an important strength parameter. In the case of a winding core, for instance, the core is usually supported at its ends and the substantial weight of a roll of web material wound on the core must be tolerated without the core failing in bending.
The axial strength of a paperboard tube can also be an important strength parameter in some cases. Yet another important strength property of paperboard tubes in some applications is the tube""s ability to withstand very high-speed rotation about its axis without failing as a result of radially outward centrifugal forces.
The various strength properties of a paperboard tube depend to a large extent on the strength properties of the paperboard plies making up the tube. In general, a paperboard tube of a given diameter and wall thickness can be increased in strength by making the tube from paperboard plies of a higher strength. Paperboard materials of various grades are commercially available. The grade of a paperboard is generally understood in the industry to correlate with the strength of the paperboard.
Laminated structures such as those described above tend to be limited in strength by the strength of the weakest link in the structure. In many laminated structures, the sheet material layers are weaker than the adhesive that binds them together. Where the structure is made up of layers of dissimilar materials, and hence different strengths, the factor limiting the strength of the structure therefore tends to be the strength of the weakest sheet material layer. Nevertheless, there may be good reasons to use such weaker layers in the structure, as opposed to using all strong layers. For example, as noted, in some cases there may be a cost advantage to using at least some weaker layers in the structure. In other cases, the weaker layer may serve another purpose that is needed and cannot be fulfilled by the other stronger layers; as an example, the weaker layer may be included because it serves as a needed fluid barrier while the other stronger layers do not. It would be advantageous to be able to reinforce such weaker layers to improve the strength of the laminated structure.
In some laminated structures constructed from layers of dissimilar materials, some layers may not be as readily bondable to the adhesive used for joining the layers together as other layers of the structure. As a result, the weak link in the structure may be the adhesive bond between such a less-bondable layer and its adjacent layer or layers. It would be desirable to be able to eliminate this weak link.
In other laminated paperboard structures, the chosen paperboard material for constructing the structure may be such that it does not bond to the adhesive as well as would be desired for optimal strength. For instance, paperboards that are densified to increase their strength sometimes tend to have poorer adhesive bonding than paperboards of lower density and strength. The strength benefit that such stronger plies provide thus can be partially offset by the lower adhesive bond strength between the plies. It would be desirable to remedy this situation.
The present invention addresses the above needs and achieves other advantages. A laminated structure in accordance with a first aspect of the invention comprises a plurality of sheet material layers joined together by an adhesive, the sheet material layers including at least one intermediate layer located other than on an outer surface of the laminated structure (i.e., located between other outwardly disposed layers). The intermediate layer preferably is formed of a material having a lower modulus or strength than that of the outwardly disposed layers. In a conventional laminated structure the anchoring of the outwardly disposed layers to each other occurs through the intermediate layer; thus, if the intermediate layer is weak, the overall strength of the structure is compromised. In accordance with the invention, however, the intermediate layer has a plurality of openings formed therein through which the adhesive penetrates so as to form adhesive bridges that extend through the intermediate layer and anchor the outwardly disposed layers to each other. Thus, the adhesive bridges reinforce the weaker intermediate layer and augment the anchoring of the outwardly disposed layers to each other.
The invention is applicable to various types of laminated structures. The sheet material layers can be formed of any suitable material, and can be rigid in some cases while in other cases they may be flexible.
In accordance with another aspect of the invention, the laminated structure comprises a paperboard structure constructed of a plurality of paperboard plies one or more of which is reinforced with the adhesive bridges. The paperboard plies can all be identical or can be of different paperboard materials. Preferably, the openings in the reinforced ply collectively have a total area making up about 2 to 25 percent of the surface area of the ply, more preferably about 5 to 20 percent of the ply surface area. Each opening can have an area from about 0.1 mm2 to about 20 mm2, more preferably about 1 mm2 to about 15 mm2.
Various sizes and shapes of openings can be formed through the ply or plies to be reinforced, such as circular holes, polygonal holes, slits, etc. In preferred embodiments of the invention, the openings comprise elongate slits (i.e., having a length dimension substantially greater than the width dimension) so that the perimeter of the opening is increased for a given opening area, or alternatively the opening area is reduced for a given perimeter. For instance, the openings can comprise rectangular holes having a length several times greater than the width. The openings preferably are oriented so that their length directions are substantially aligned with the direction along which the largest tensile loads are expected to be placed on the paperboard plies during a particular application. In this manner, the openings result in a relatively smaller reduction of the cross-sectional area of the ply that is available to support the tensile loads, compared to alternative orientations of the openings. It is further beneficial to stagger the openings in the lengthwise direction of a ply so as to limit the number of openings aligned along any given line extending in the widthwise direction of the ply; this also helps maximize the cross-sectional area of the ply available to support the tensile loads.
The openings in a ply or plies can be provided in various ways. The paperboard could be produced on a paper machine such that the paperboard comes off the machine having a substantial porosity or having small holes such that adhesive can penetrate through it. The openings alternatively can be made by pricking or puncturing a formed ply with a sharp tool or punch die.
Each ply having the openings preferably is disposed between two other plies, which may or may not have openings. Preferably, the plies forming the outer surfaces of the laminated structure do not have openings.
In some preferred embodiments, plies having openings are alternated with plies not having openings. In other embodiments, a plurality of contiguous plies each having openings can be incorporated in the laminated structure. Where two contiguous plies both have openings, it is considered preferable but not essential to stagger the openings so that the openings in one ply are not aligned with the openings in the adjacent ply.
The invention is applicable to multi-grade paperboard structures such as paperboard tubes. In particular, one or more lower-strength paperboard plies can be incorporated into a tube and can be reinforced by openings with adhesive bridges. Thus, multi-grade paperboard tubes formed of higher-strength and lower-strength paperboard plies can be improved in strength, relative to equivalent tubes not having openings through the lower-strength plies. In preferred embodiments of the invention, about 5 to 95 percent, more preferably about 30 to 70 percent, of the wall thickness of a multi-grade paperboard tube is made up of lower-strength paperboard plies, and some or all of the lower-strength plies have openings therethrough. The lower-strength plies having openings can be alternated with higher-strength plies, which may or may not have openings; alternatively, a plurality of contiguous lower-strength plies having openings can be included in the tube.
When a lower-strength ply is provided with openings and adhesive bridges are formed through the ply linking together stronger plies on either side of the weak ply, the resulting structure""s strength is no longer limited by that of the weak ply, or at least is limited to a lesser extent than it otherwise would be.
The openings and adhesive bridges can also be advantageous in paperboard structures formed of densified high-strength plies, or formed with treated (e.g., saturated or coated) paperboard, which tend to bond with adhesive less tenaciously than may be desired. The adhesive bridges can be provided through the plies to form a continuous adhesive lacing or webbing to provide added strength to the ply interfaces.